1) Field of the Invention
The present invention relates to a transfer apparatus, an image forming apparatus, and a method of belt-speed correction, in which an actual speed of a belt is detected by reading a scale that is provided along the whole circumference of the running belt, by a sensor, and performing a correction control by applying correction to a belt speed that is detected by the sensor to match the belt speed with a target speed.
2) Description of the Related Art
In recent years, a majority of image forming apparatus in which electrophotography is employed, like copying machines and printers, are capable of forming a full-color image as per the requirement of the market.
Some color image forming apparatuses, called one-drum (single drum) image forming apparatuses, include a plurality of developing units around one photosensitive drum, that develop images with toners of different colors. The developing units apply toners on a latent image on the photosensitive drum to form a super-imposed full color toner image. The toner image is transferred to a sheet that is a recording material to obtain a color image.
Among the color image forming apparatuses, there are apparatuses, called tandem image-forming apparatuses, in which a plurality of photosensitive drums are arranged in a row and a developing unit is provided for each of the photosensitive drum, where each developing unit develops different color. A single color toner image is formed on each of the photosensitive drums and these single color toner images are transferred one after another to a belt or a sheet. Thus, a full color toner image is formed on the belt or the sheet.
The one drum image forming apparatuses are smaller because there is only one drum. The smaller size also implies reduced cost which is another advantage. However, in the one drum image forming apparatus, the photosensitive drum is rotated several time (four time for a full-color image) to form one full-color image which leads to difficulties in accelerating the speed of image formation. This is a drawback of the one drum image forming apparatus.
Whereas, in the case of the tandem image-forming apparatus, due to the requirement of plurality of photosensitive drums, the image forming apparatus tend to be bigger, consequently resulting in increase in the cost. However, there is a scope for accelerating the speed of image forming in the tandem image-forming apparatus.
Lately, a color image is also sought to be formed with the speed same as that of the monochrome image formation and the tandem image-forming apparatus has been drawing attention for this reason.
As shown in FIG. 17, the tandem image-forming apparatus includes photosensitive drums 91Y, 91M, 91C, and 91K that are arranged in a straight line. Toner images on the photosensitive drums 91Y, 91M, 91C, and 91K are transferred one after another to a sheet P that is carried on a sheet carrier belt 93 which is running in a direction of an arrow A, by transferring units 92 for each photosensitive drum. Thus, a full-color image is formed on the sheet P by a direct transfer. As shown in FIG. 18, toner images on a plurality of photosensitive drums 91Y, 91M, 91C, and 91K are transferred one after another to an intermediate transfer belt 94 that is running in a direction of an arrow B such that the images are superimposed. The superimposed image on the intermediate transfer belt 94 is transferred collectively to a sheet P by a secondary transferring unit 95. Thus, the superimposed image is formed on the sheet P by an indirect transfer.
If the direct transfer and the indirect transfer are compared to each other, in the former case, the plurality of photosensitive drums is arranged in a line. A paper feeding unit 96 is disposed on an upstream side of the drum arrangement and a fixing unit 97 is disposed on a downstream side of the drum arrangement. This structure results in increase in size along the direction of sheet transfer.
Whereas in a case of the indirect transfer, a secondary transferring position can be set voluntarily. Therefore, as shown in FIG. 18, the secondary transferring unit 95 is disposed on a downstream side of the intermediate transfer belt 94 and a paper feeding unit 96 can also be disposed on the downstream side of the intermediate transfer belt 94. Therefore, it is possible to reduce the size of the image forming apparatus along the width of the apparatus (a direction from left to right in FIG. 18).
In a direct-transfer tandem image-forming apparatus, if the size of the apparatus is reduced to a possible extent along the width of the apparatus, the fixing unit 97 has to be disposed close to the sheet carrier belt 93. If the fixing unit 97 is disposed, close to the sheet carrier belt 93, when a tip of the sheet P reaches a nip of the fixing unit 97, even if the sheet P tends to bend due to the difference in linear speeds of the sheet carrier belt 93 and the fixing unit 97 (linear speed of the fixing unit 97 is slower), since the distance from the sheet carrier belt 93 to the fixing unit is extremely short, particularly in a case of a thick sheet, the sheet vibrates due to striking when a tip of the sheet reaches the nip of the fixing roller 97. This may result in affecting the image.
To solve this problem, in the case of an indirect-transfer tandem image-forming apparatus, since the secondary transferring unit 95 can be disposed on the downstream side of the intermediate transfer belt 94, even if the size of the apparatus is reduced along the width of the apparatus, the fixing unit 97 can be disposed such that it is away from the intermediate transfer belt. As a result, even when the tip of the sheet reaches the nip of the fixing unit 97, the sheet bends so that it can overcome the difference in the linear speeds of the intermediate transfer belt 94 and the fixing roller 97. Thus, the difference in the linear speeds is overcome, thereby avoiding any effect on the image.
Due to these various advantages, the indirect-transfer tandem image-forming apparatus has been drawing attention nowadays.
However, in the tandem image-forming apparatus, where a plurality of photosensitive drums are arranged in a line to deal with toners of different colors, the color image is formed by superimposing the toner images of different colors that are formed on the photosensitive drums on a sheet or on an intermediate transfer belt. If the positions of superimposing of the toner images of different colors are shifted, there is a shift in colors or a minute change in shade which deteriorates the image quality. Therefore, position shift (color shift) of the toner image of each color has been an important issue.
One of the known causes of the color shift is an uneven speed of the intermediate transfer belt in the case of the indirect transfer (uneven speed of the sheet carrier belt in a case of the direct transfer).
According to a technology disclosed in Japanese Patent Application Laid-open Publication No. H11-24507, an arrangement to apply correction to the uneven speed of the intermediate belt is provided in a color image forming apparatus that uses a conventional transfer belt.
A color copying machine that is disclosed in Japanese Patent Application Laid-open Publication No. H24507, includes an intermediate belt (the transfer belt) that is stretched over five supporting rollers that include a driving roller. Toner images of four colors viz. cyan, magenta, yellow, and black are transferred on an outer circumferential surface of the intermediate transfer belt by superimposing the image for each color one after another. Thus, a full-color image is formed.
An inner surface of the intermediate transfer belt of the color copying machine is provided with a scale having minute and precise graduations. The scale is read by an optical detector to detect the speed of the intermediate transfer belt accurately. The detected speed is subjected to feed-back control by a feed-back control system to adjust the correct speed of the intermediate transfer belt.
The feed-back control system is equipped with a position controlling circuit (position controller), a speed controlling circuit (speed controller), a power converting circuit (power converter), a position detecting circuit (position detector), a speed detecting circuit (speed detector) etc. apart from the optical detector. The position controlling circuit calculates deviation of the accurate and minute position signal from the position detecting circuit (position detector) from a target position of the intermediate transfer belt and calculates the target speed of the intermediate transfer belt. The position controlling circuit outputs the target speed calculated to the speed controlling circuit. The speed controlling circuit calculates deviation of a speed signal that is input from the speed detecting circuit from the accurate target speed that is input from the position controlling circuit. The speed controlling circuit then calculates an accurate power that is to be supplied to a motor that drives the intermediate transfer belt and outputs the calculated power signal to the power converting circuit. Thus, by controlling the drive of the motor, the speed of the intermediate belt is adjusted accurately.
However, to carry out the accurate correction control to adjust the speed of the intermediate transfer belt to the target speed by detecting the speed of the intermediate transfer belt and carrying out a feed-back control, a feed-back control system that is equipped with a highly precise speed detecting system is required. Realization of such feed-back system results in an increase in the cost of the unit.
A small fluctuating frequency component of high frequency that is developed during operation of the image forming apparatus and fluctuating frequency component of low frequency that is developed due to change in the belt speed slowly are factors that contribute to a fluctuation in speed of the intermediate transfer belt (same in the case of the sheet carrier belt). The combination of the fluctuating frequency components of low frequency and the fluctuating frequency component of high frequency appear as fluctuation in speed of the belt. If all the factors that cause the fluctuation in speed of the belt are to be detected and if the correction is to be applied to all the factors, it is necessary to have a highly precise speed detecting system. Such system would be a very complicated one to be realized and will result in a very high rise in cost.